A nut cooperating threadably with a helically threaded screw can be adjusted to specific locations axially along the screw, by and upon relative rotation of the nut and screw. Axial forces can further be generated by the axial advance of the nut along the screw, but as the axial forces are increased, the friction between the cooperating threads is also increased, making nut rotation on the screw more difficult and only upon using higher torquing forces.
In order to overcome friction between the sliding cooperating threads, commercial linear actuators utilize a ball-screw mechanism that axially keys the screw and follower together while providing a rolling support therebetween. Such linear actuators will operate on reduced torque in providing relative screw-follower rotation and/or will provide for more efficient or easily generated axial forces or for smoother axial/rotational movements between the screw and follower, compared to direct sliding threads that generate high thread friction.
However, such typical linear actuators also provides several challenges or drawbacks, including: (1) the cooperating thread profile must be modified so that bearings balls can be fitted between the threads on the screw and follower, being more costly and/or more difficult to inventory to satisfy different specific applications; (2) the follower is free wheeling on the screw, requiring that many applications might need some form of locking mechanism to hold the instantaneous actuator position fast in the event of loss of actuating electrical power or the like; (3) the ball contact with the adjacent threads is across a point or small localized area, generating concentrated mechanical stresses against the threads that might damage them unless such were made of costly hardened materials, particularly when the linear drive is generating large axial forces between the screw and follower; and (4) routing mechanism spaced from the threads is needed to return the bearing balls from one follower end to the other to follower end, providing for an endless availability of balls to have only ball rotation between the threads, but again adding both size and costs to the linear drive.
The invention to be disclosed herein deviates from the noted approach, and advantageously utilizes the fact that friction between sliding threads of cooperating screw and nut components, and the force needed to turn the nut on the screw, increase and decrease as the axial load or force between the component threads are respectively increased and decreased.